[CI] check the return value of `rally task`
[CI] check the return value of `rally task`

The value 2 is sla error, the ci should continue to work.
It would be easy to find out the failed workload via `rally
task sla-check`.

Change-Id: I950c82353d88bc89a96ce3c9481ee45571d5d2ba

openstack/rally,openstack/rally,openstack/rally,openstack/rally